In the drive for modern and efficient methods of dimensional control and surface processing, a process which is under-utilised is electro-polishing. Electropolishing, also known as electrochemical polishing, electrolytic polishing, or electrolytic etching, electrochemical machining (ECM), or electrochemical material dissolution is a process that removes material from the surface of a metallic article, usually maintaining a physical stand-off or separation of a tool relative to the surface of the article. Such tools are commonly shaped according to specific contours or shapes required of the finished article.
Electropolishing is typically utilised in the metal finishing industry for applications requiring, for example, one or more of polishing, etching, passivating and deburring metallic components. In particular, electropolishing can be used to machine articles of complex geometry and/or materials which are otherwise difficult to machine, with high degrees of accuracy and without imparting the levels of residual stress or mechanical deformation of surface layers observed when using traditional machining methods. As such, electropolishing provides the advantage of providing clean and smooth surfaces, allowing the capability to reduce the height or depth of micro peaks and valleys, whilst providing localised material removal material in pre-determined areas.
Whilst providing several advantages, there are known issues associated with electropolishing, particularly in detecting non-uniform metal removal. Here, the differences in surface profile are most typically fairly subtle, often being in the order of tens of microns or less and so challenging the resolution and capabilities of available geometrical inspection methods. For hot isostatic pressed (HIP) components formed from powder constituents, it is known that regions of remnant canister inter-diffused material may remain following electropolishing. The described areas may look visually similar in terms of colour, roughness and reflectivity, making these regions of non-compliant chemistry hard to detect.
Electropolishing is also used in the removal of material from articles of complex geometry including, for example, non axi-symmetric features such as bosses, internal and external corners, and sharp changes in section. As such, electropolishing may be used in the manufacture of pressure vessels and/or aerospace articles such as disks, drums, bladed discs and bladed drums. When considering such articles, such complex geometries may lead to field concentrating features, which may in turn lead to anisotropic material removal. Here, slight deviations in article geometry may lead to such field concentrating or diluting features removing more or less material than would otherwise be required. Such anisotropy can be difficult to detect and rectify following treatment as an in-process monitoring system which seeks to monitor local effects is not presently known. Instead, known systems tend to operate at resolutions corresponding to the scale of the entire article rather than the scale of the material removal itself. Accordingly, the level of removed material is difficult to quantify when operating at low orders of magnitude.
In addition, checking for retained canister material, material defects, or non-process conformance using non-destructive methods is challenging, particularly on articles of complex geometry including areas of rapidly changing contour features. Thus, the ability to test material removal in areas associated with an increased risk of non-conformance using economically viable methods is challenging.
It would therefore be advantageous to provide a reliable apparatus and method for the electropolishing of an article of complex geometry by incorporating an associated means of process control without the aforementioned disadvantages.